Conventionally, a thermal protector is structured to shut down a conduction path by the inversion operation of a bimetal element. Then, the bimetal element itself or a movable plate jointed to the bimetal element forms a conduction part for shutting down the conduction path.
Therefore, the thermal protector is structured so that the bimetal element part is always heated by itself with Joule heating in a current path where current flows from one terminal to the other terminal regardless of the position of a contact for shut-down.
Therefore, the bimetal element is operated by not only ambient temperature but also the influence of Joule heating generated by the bimetal element itself. Thus, an inconvenience that the shutting operation is caused at lower ambient temperature in which the shutting operation is not needed is often seen.
Therefore, in order to avoid such an inconvenience, a thermal protector structure in which no conduction part is formed in portions other than a contact part of the bimetal element is proposed (for example, see Japanese Patent No. 3724178 (Japanese Laid-open Patent Publication No. H11-260221).
FIG. 1 is a perspective view showing the structure of such a thermal protector for forming no conduction part in portions other than the contact part of the bimetal element.
As illustrated in FIG. 1, in this thermal protector 1, two plane-shaped fixed electrodes 2 and 3 go through the lower section of a resin base 4 being a support member from front to rear and are supported by the resin base 4.
At one ends of the two fixed electrodes 2 and 3, fixed contacts 5 and 6 are formed and to the other ends of the two fixed electrodes 2 and 3 projected from the resin base 4 opposed to the fixed contacts 5 and 6, lead wires 7 and 8 are connected.
On the surface of the resin base 4 positioned above the end side having the fixed contacts 5 and 6 of the two fixed electrodes 2 and 3, one end of a movable electrode support plate 9 is fixed. Then, to this movable electrode support plate 9, one end of a bimetal element 10 to be inverted by heat is fixed and the bimetal element 10 is supported.
Then, at the other end of the bimetal element 10, one movable contact 11 is provided opposed to the fixed contacts 5 and 6.
In this thermal protector 1, as illustrated in FIG. 1, the movable contact 11 of the bimetal element 10 is contacted on the fixed contacts 5 and 6 by pressure at normal temperature. Thus, a conduction path is formed between the lead lines 7 and 8 via the fixed electrode 2, the fixed contact 5, the movable contact 11, the fixed contact 6 and the fixed electrode 3 in that order.
Then, the bimetal element 10 is structured in such a way that the bimetal element 10 may be inverted at ambient temperature equal or more than prescribed temperature, the movable contact 11 may be separated from the fixed contacts 5 and 6 and the conduction path formed between the lead lines 7 and 8 may be shut down.
However, as clearly seen in FIG. 1, the fixed electrodes 2 and 3 between the fixed contacts 5 and 6 and the resin base 4 are conduction areas and these conduction areas are disposed opposed to the bottom surface of the bimetal element 10.
Specifically, the entire surface of inversion area of the bimetal element 10, that is, 100% of the inversion area overlaps the conduction areas of the fixed electrodes 2 and 3.
In this way, although the bimetal 10 is structured not to be energized, in other words, the bimetal element 10 itself is structured not to generate heat by Joule heating, the entire inversion area of the bimetal element 10 is in such a state as to receive Joule heating generated in a conduction area by radiation and convection.
Therefore, when conduction current increases, the bimetal element 10 is inverted by not only ambient temperature but also heat generated inside the thermal protector 1 itself and the thermal protector 1 tends to be frequently operated at lower ambient temperature than its original operating temperature.
When the conduction current further increases, as described above, the thermal protector illustrated in FIG. 1, the bimetal element 10 can be inverted at normal temperature.
Specifically, practically, the thermal protector 1 is structured in such a way that there is a possibility that the thermal protector 1 may be wrongly operated despite being at ambient temperature in the usual operation range of the device when being incorporated into a device.
Accordingly, it is an object of the invention to provide a thermal protector capable of conducting large current by minimizing the influence of heat generation by conduction.